ALBERT

Description: At present, three continental archetypal ichnofacies are widely accepted: the Scoyenia, Mermia and Coprinisphaera ichnofacies. The last is present in palaeosols, and the first two occur in fluvio-lacustrine environments. Additionally, the Skolithos ichnofacies may be present in relatively high-energy fluvio-lacustrine deposits. The ichnofauna from active fluvial channels is characterised by low-diversity assemblages of simple vertical burrows and escape traces, referred to the Skolithos ichnofacies. Abandoned or inactive channel deposits characteristically contain low-diversity assemblages dominated by meniscate traces. Floodplain water bodies that experienced progressive drying (desiccated overbank deposits) may contain abundant arthropod and vertebrate trackways, backfilled meniscate traces, ornamented burrows and bilobate traces with scratch marks, which allow recognition of the Scoyenia ichnofacies. Floodplain water bodies that are filled by overbank vertical accretion without experiencing desiccation (overfilled overbank deposits) include simple grazing trails, locomotion trails and horizontal dwelling burrows, representing impoverished occurrences of the Mermia ichnofacies. Hydrologically closed lakes are very stressful environments in which subaqueous ichnofaunas are rare. The richest ichnofaunas in closed lakes are present at the lake margins, and record the activity of terrestrial rather than aquatic faunas (Scoyenia ichnofacies). Hydrologically open lakes host relatively diverse and abundant ichnofaunas, comprising the Scoyenia ichnofacies in low-energy, lake-margin areas, and the Mermia ichnofacies in permanent subaqueous lacustrine zones. Sediments deposited in relatively high-energy lacustrine environments, such as wave-dominated shorelines and delta mouth-bars, commonly are represented by the Skolithos ichnofacies. Although continental trace fossils have not been extensively used in sequence stratigraphy, they have potential for future integrated study. Softground trace fossils are commonly well developed in overfilled lake basins and are useful to delineate parasequences and parasequence sets. In balanced-fill and underfilled lake basins, softground ichnofaunas are poorly developed because of stressful conditions. In contrast, firmground suites are rare in overfilled lake basins, but widespread in lowstand deposits of balanced-fill and underfilled lake basins. Early lowstand amalgamated incised fluvial channels are usually unbioturbated, but palaeosol ichnofaunas (e.g. the Coprinisphaera ichnofacies) may delineate sequence boundaries in interfluve areas. Increasingly isolated fluvial channels encased in overbank deposits develop during late lowstand, and lacustrine deposits accumulate during transgressions, favouring preservation of biogenic structures.

Description: Trace fossils are sensitive indicators of environmental fluctuations and, accordingly, ichnological studies have the potential to improve facies characterization of marginal-marine systems. Carboniferous intertidal deposits in eastern Kansas and western Missouri accumulated under contrasting palaeoenvironmental conditions, ranging from the open shoreline to fluvio-estuarine transitions. Comparative analysis of these exposures illustrates lateral variations in trace-fossil content and allows characterization of the intertidal ichnofaunas developed in three sub-environments: open marine, restricted bays and fluvio-estuarine transitions. Openmarine tidal flat ichnofaunas are characterized by (1) high ichnodiversity, (2) marine ichnotaxa produced by both euryhaline and stenohaline forms, (3) the presence of both infaunal and epifaunal traces, (4) the presence of simple and complex structures produced by presumed trophic generalists and specialists respectively, (5) dominance of horizontal trace fossils of the Cruziana ichnofacies, (6) presence of multispecific associations, (7) high density, and (8) wide size range. This ichnofauna is present in heterolithic deposits and reflects the activity of a biota that inhabited tidal flats dominated by normal-marine salinities and connected directly to the open sea. Restricted-bay ichnofaunas display (1) low ichnodiversity, (2) ichnotaxa commonly found in marine environments, but produced by euryhaline organisms, (3) dominance of infaunal traces rather than epifaunal trails, (4) simple structures produced by opportunistic trophic generalists, (5) a combination of vertical and horizontal trace fossils from the Skolithos and Cruziana ichnofacies, (6) the presence of monospecific associations, (7) variable density, and (8) small size. This assemblage occurs in heterolithic facies and records the activity of a brackish-water benthic fauna inhabiting intertidal areas of estuarine basins and embayments. Fluvio-estuarine ichnofaunas are characterized by (1) moderate to relatively high diversity, (2) forms typically present in continental environments, (3) the dominance of surface trails and absence of burrows, (4) temporary structures produced by a mobile deposit-feeding fauna, (5) a mixture of trace fossils belonging to the Scoyenia and Mermia ichnofacies, (6) moderate density of individual ichnotaxa, (7) absence of monospecific suites, and (8) small size. This assemblage occurs in tidal rhythmites and records the activity of a typical freshwater/terrestrial benthos inhabiting tidal flats that were developed in the most proximal zone of the inner estuary under freshwater conditions. Through integration of ichnological and sedimentological data, conventional sedimentological interpretations of marginal-marine depositional systems can be refined and enhanced.

Description: Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8 billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992) Science 257:232–235; Knoll et al. (2006) Philos Trans R Soc Lond B 361:1023–1038], Stirling biota [Bengtson S et al. (2007) Paleobiology 33:351–381], and large colonial organisms exhibiting signs of coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we report on pyritized string-shaped structures from the Francevillian Basin. Combined microscopic, microtomographic, geochemical, and sedimentologic analyses provide evidence for biogenicity, and syngenicity and suggest that the structures underwent fossilization during early diagenesis close to the sediment–water interface. The string-shaped structures are up to 6 mm across and extend up to 170 mm through the strata. Morphological and 3D tomographic reconstructions suggest that the producer may have been a multicellular or syncytial organism able to migrate laterally and vertically to reach food resources. A possible modern analog is the aggregation of amoeboid cells into a migratory slug phase in cellular slime molds at times of starvation. This unique ecologic window established in an oxygenated, shallow-marine environment represents an exceptional record of the biosphere following the crucial changes that occurred in the atmosphere and ocean in the aftermath of the great oxidation event (GOE).

Description: Although the notion of recurrent trace-fossil assemblages through geologic time as a result of similar sets of environmental conditions has a long history in ichnology, the idea that recurrent ichnofaunas may have occurred in connection with macroevolutionary events has not yet been put forward. We refer to this phenomenon as "the Deja vu effect," and suggest that understanding its significance may shed light on how organisms colonize underutilized or empty ecospace, the establishment of the mixed layer, and the temporal and spatial distribution of matgrounds. These recurrent trace-fossil assemblages record the activity of the epifauna and a very shallow tier infauna, in the absence of mid- to deep-tier trace fossils and mottled textures. They typically consist of grazing trails, very shallow feeding burrows, and arthropod trackways. Shallow-tier burrows and trails with well-developed scratch marks may be present. These ichnofaunas suggest that the initial exploitation of underutilized or empty ecospace is linked to a set of recurrent behavioral strategies to obtain food from matgrounds and firmgrounds in the absence of a well-developed mixed layer. Preservation of these behaviors in the form of trace fossils is mediated by a set of taphonomic conditions. These taphonomic windows occur during the initial colonization of empty or underutilized ecospace, such as the Ediacaran-Cambrian transition or the subsequent colonization of the land, but also after the end-Permian mass extinction, which may have had a negative impact on the mixed layer in marine environments, producing a return to the ecologic conditions of the early Paleozoic.

Description: Widespread microbial mats and the absence of significant bioturbation resulted in a poorly developed mixed layer and extensive cohesive substrates during the Ediacaran-Cambrian transition. Large nonbiomineralized arthropod carapaces overprinted with trails, interconnected burrow systems, narrow-caliber structures with dendritic terminations, and annulated burrows are abundant in the Early Cambrian Sirius Passet Lagerstätte (Greenland). Taphonomic controls were partially responsible for the pronounced association of these structures and carapaces, but ecologic conditions are envisioned as playing a significant role. Although some trace fossils (e.g., annulated structures) may be related to scavenging and/or deposit feeding, morphologic evidence suggests that this is not always the case. Interconnected burrow systems provide evidence for re-use of structures, suggesting grazing on or farming of bacteria. Sulfur bacteria were probably abundant in Cambrian sediments, requiring a source of sulfur or sulfide and limited quantities of oxygen. Anoxic sediment provided a source of hydrogen sulfide, whereas burrow outlets connected to the oxygenated sediment-water interface. Carcasses and molds of nonbiomineralized organisms may have acted as attractors, supporting a community of small invertebrates. Carapaces created a sharp boundary, further decreasing oxygen diffusion into the sediment. This steep oxygen gradient at the organic surface of the carapace promoted the growth of sulfur bacteria that could have served as the primary food for small macrofauna and meiofauna.

Description: During forced regression, the coastline advances seaward irrespective of sediment supply and relocates to a topographically lower position. Forced-regressive deposits are a component of the falling-stage systems tract. They are recognized by a basal unconformity which records erosion during the seaward facies shift, and are in turn capped by another unconformity due to subaerial exposure or wave ravinement during subsequent transgression. In wave-dominated settings, the basal “regressive surface of marine erosion” is a scoured surface cut by wave action, because erosion is needed to maintain the seaward-sloping bathymetric profile in equilibrium with the wave energy. The response to falling sea level in tide-dominated settings has not been as thoroughly discussed. The stratal architecture of the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains reveals a new mechanism for the formation of this surface landward of the lever point of balance between sedimentation and erosion in the subtidal environment. Subtidal parasequences of the Lake O'Hara Member are typically composed of thin-bedded mudstone and ripple cross-laminated sandstone, followed by Skolithos pipe rock and thin- to medium-bedded cross-stratified sandstone overlain by thick-bedded cross-stratified sandstone. However, at the top of the Lake O'Hara Member, an erosional surface truncates a pipe-rock interval and the expected capping cross-stratified sandstones are absent. Overlying this surface, the Lake Oesa Member comprises tidal-flat deposits composed mainly of thinly interbedded mudstone and sandstone. This jump in facies at the erosional surface can be explained as a response to a fall in sea level. As the shoreline is forced to regress, the laterally continuous tidal flats advance and the pre-existing shallow-subtidal compound dunes are scoured by strong tidal currents, along with waves, that gradually carve a new equilibrium profile. We argue that the accretion of intertidal flats on top of subtidal sands is an overlooked yet predictable component of falling-stage systems tracts in tide-dominated settings.

Description: Integrating sedimentological and ichnological characteristics of the Gelasian Mayaro Formation along the SE coast of Trinidad allows recognition of a river-dominated and wave-influenced shelf-edge deltaic succession of the paleo–Orinoco River developed under strong slope instability. Four main sedimentary settings and twelve subenvironments developed on and beyond the outer shelf (i.e., at the shelf margin) have been identified. Extreme paleoenvironmental conditions make the succession rarely and sporadically colonized. Ichnologic evidence suggests that shelf-edge deltas are among the most stressful marine environments, due to a combination of physicochemical factors in response to the intrinsic sedimentary processes and the relative hierarchy of their influences specific to every subenvironment. Within any particular subenvironment, the relative dominance of the fluvial feeder system, the action of waves (and rare influence of tides), and slope instability determine the combinations and ranking of stress factors. River-dominated shelf-edge delta subenvironments demonstrate the extreme influence of stress factors related to channel activity (e.g., salinity changes, high sedimentation and erosion rates, flocculation of mud, seasonal variations of fluvial flux), whereas subenvironments dominated by waves, storms, and oceanic swells away from the axial feeder(s) are mostly influenced by barform morphodynamics and its effects on local physiography. The paleo–Orinoco delta is represented not only by the delta lobe(s) developed at the shelf edge but by delta lobe(s) formed on the outer shelf as well. Outer shelf deltaic subenvironments manifest typical ichno-sedimentological signatures of a “normal” (wave-influenced in this case) inner-shelf delta. Being susceptible to extensive gravitational instabilities, the delta system suffered from canyonization (and smaller gullying) and subsequent filling (N.B. Hereafter we use the term canyon for both canyons and smaller gullies). The shelf-edge delta system was also associated with “shelf-attached” mass-transport systems. Such depositional subenvironments dominated by slope instability and sediment-gravity-flows appear to be the most unconducive for benthic colonization, resulting in almost complete absence of bioturbation. Characterization of the stress factors, as the functions of parameters arising from specific sedimentary and/or oceanographic processes, chemical conditions, and preservational constraints specific to each subenvironment, leads to the construction of a comprehensive ichnological and depositional analog model for shelf-edge deltas in general, and for the accommodation-driven low-latitude shelf-edge deltas at an active oblique foreland setting in particular.